A lithium battery is made from three main components: anode, cathode and electrolyte.
Anode and cathode normally consist of metal foils which are covered by a thin layer of a powder mixture, active materials, which are bound together by a binder. The binders function is to glue the powder particles together and glue these firmly to the metal foil. The binder must be flexible and chemically stable towards the electrolyte.
A typical anode consists of a copper foil which is coated by a thin layer (40-100 microns) with graphite powder, carbon, which is tied together by means of the plastic material PVDF (polyvinylidene fluoride).
A typical cathode consists of an aluminum foil which is coated by a thin layer (40-100 microns) of lithium metal oxide which is bound together by the plastic material PVDF.
A typical electrolyte is a mixture of a lithium salt such as lithium hexafluorophosphate (LiPF6), lithium tetrafluorophosphate (LiPF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), Lithium tetrafluoroborate (LiBF4), and lithium triflate (LiCF3SO3) and organic carbonates, for instance EC (ethylene carbonate), DEC (diethyl carbonate) and DMC (dimethyl carbonate).
The most common manufacturing process for making a battery film for lithium ion batteries is to blend active materials and PVDF, and mix this into a solvent dissolving the PVDF. The purpose to dissolve the binder is to disperse the material evenly between the particles in the powder mixture in order to secure a good binding between these. This mixture is then applied to the metal foil by means of extrusion, rolling or tape-casting depending on selected process and amount of solvents used. After application the foil will be dried by evaporation of solvents.
The most common solvent in order to dissolve PVDF is NMP (N-Methyl-Pyrrolidone), which is both a toxic and environmentally harmful chemical. There are a variety of alternative solvents, but most of them have in common that they are either toxic, liable to catch fire or unfavorable relating to the chemical structure of the finished battery. Consequently, it is important that the solvent is fully removed from the battery film during production and that the evaporation of NMP is controlled with regards to the environmental requirements. The process of removing the last remnants of the solvent (down to ppm level) from the battery film is a demanding process which is both energy- and space-demanding and makes substantial demands from the technical equipment.
There are waterborne manufacturing processes in which the powder is mixed with water to form a paste or thin slurry. The disadvantage of using water is the relatively energy demanding process to evaporate the water so that the dried battery film must be completely free from water so that the battery shall operate.
From US 2005/0271797 A1 it is known that a production process for a lithium battery consisting of the steps of a) prepare an EC (ethylene carbonate) solution by loosening EC-crystals in a suitable solvent, (b) then dissolve a binder in a suitable solvent in order to make a binder solution and then add and mix sufficiently an active electrode material and an electric conductive material of a wanted composition into the binder solution, (c) add a defined amount of the EC mixture prepared in step (a) into the binder solution from step (b), blend the mixture of EC solution and the binder solution sufficiently so that the slurry in form of an electrode binder can be coated onto an electrode, (e) coat a collector with the slurry, (f) dry the paste layer at a given temperature, and complete the electrode production by pressing a dried electrode structure at a given pressure after the slurry has dried. The process described in US 2005/0271797 A1 also comprises mixing a solvent together with an ethylene carbonate plus insert a second solvent to a binder solution together with an active material for then subsequently to add a given amount of the solvent mixed with ethylene carbonate to the mixture of the binder solution with the other solvent. Thus the process comprises the application of at least one solvent for generating the slurry.
There is a need for providing a manufacturing process of slurry for electrode materials for lithium batteries which is not burdened with the problems associated with use of solvents indicated above.